Expectations are rising regarding the role of natural gas as a primary energy source. It is abundant, affordable, and effective for reducing CO2 emissions compared to other fossil fuels.
The first step of the natural gas liquefaction process typically involves removal of impurities like dust, acid gases, helium, water, or heavy hydrocarbons (e.g., particularly those that would freeze prior to the natural gas liquefying). The natural gas is then condensed into a liquid by cooling it to a temperature as low as −162° C.
In previous methods for mid to large scale liquefaction of natural gas, a refrigerant loop (typically nitrogen or a mixed of hydrocarbons) is used. These methods usually have a low operating expense; however, the investment is high. For example, a closed loop nitrogen liquefier requires a significant number of pieces of equipment such as a cycle compressor, process gas coolers, two nitrogen turbo-expanders, a main heat exchanger and a nitrogen refrigerant storage. In cases where a small to very small Liquefied Natural Gas (LNG) production is desired and sufficient quantities of liquid nitrogen (LIN) are available nearby, one solution is to use the LIN as the cold medium to liquefy the natural gas in a dedicated exchanger. In this case, LIN is vaporized by heat transfer with the condensing natural gas stream. While this method of producing LNG has a low capital investment, the major drawback of this method is the high operating expense as the liquefaction of natural gas with liquid nitrogen is inefficient from a thermodynamics point of view. See FIG. 1, which shows the irreversible heat losses for liquefying natural gas against LIN in which the natural gas flow is 80 stpd at a pressure of 22 bara and the liquid nitrogen flow is 173 stpd at 4 bara.
As is clearly shown in FIG. 1, the irreversible heat loss, which is shown as the area between the two curves, is quite large. Moreover, there are other inefficiencies that should be taken into account when producing LIN and delivering it to the LIN to LNG site, such as boil-off and flash losses related to the LIN storage and LIN losses when loading or unloading the truck. Therefore, there is clearly a need for a method and device that would allow for a more efficient method of liquefying natural gas, particularly on a small scale compared to the methods described above.